1. Field of the Invention
The present invention relates to an improved method for joining tubes of a heat exchanger to an array of fins for the purpose of assembling a heat exchanger. More particularly, this invention relates to an improved method for mechanically joining tubes and fins, in which the tubes are deformed using an external expansion technique that does not involve intrusion into the tube, but instead entails the use of an expansion tool adapted to be inserted between tubes within the tube and fin assembly.
2. Description of the Prior Art
Heat exchangers are widely used in various industries in the form of radiators for cooling motors and engines, condensers and evaporators for use in air conditioning systems, and heaters. In their most simple form, heat exchangers include one or more passages through which a fluid flows while exchanging heat with the environment surrounding the passage. In order to efficiently maximize the amount of surface area available for transferring heat between the environment and fluid, the design of a heat exchanger is typically of a tube-and-fin type containing a number of tubes which thermally communicate with high surface area fins. The fins enhance the ability of the heat exchanger to transfer heat from the fluid to the environment, or vice versa.
Various heat exchanger designs are known in the prior art. Design variations include the manner in which the fluid passage is constructed and the type of fin used. For example, the passage may be composed of a single and integrally-formed ("continuous") serpentine tube that traverses the heat exchanger in a circuitous manner, or a number of discrete parallel tubes joined, typically brazed, to and between a pair of headers. An advantage with continuous serpentine tubes is avoiding the necessity to form numerous leak-proof joints between the tubes and headers and between the tubes and their interconnecting return bends (elbows) and connector tubes. The fins may be provided in the form of panels having apertures through which the tubes are inserted, or in the form of centers that can be positioned between adjacent pairs of tubes.
Conventionally, heat exchangers are manufactured by joining the tubes and fins using a brazing operation or a mechanical expansion technique. Mechanical expansion techniques rely solely on the mechanical joining of the components of the heat exchanger to ensure the integrity of the heat exchanger. As a result, advantages of mechanical expansion assembly techniques include good mechanical strength and avoidance of joining operations that require a furnace operation. However, disadvantages of such techniques include inferior thermal performance due to inadequate contact between the tubes and fins, resulting in reduced heat transfer efficiency. Accordingly, improvements in mechanical expansion techniques have often been directed to ways in which the integrity of the tube-to-fin joint can be improved.
Conventional mechanical expansion methods can generally be categorized as being external or internal operations. Internal expansion techniques typically entail forcing an expansion tool into the tubes to physically force the walls of the tubes outward and into engagement with the fins. In contrast, external expansion techniques have generally entailed deforming the tubes with an expansion tool that impacts or presses the tubes into engagement with the fins. While internal expansion methods tend to be characterized by enhanced joint strength and a lower resistance to heat transfer, the intrusion of a tool into the tubes is generally undesirable from the standpoint of the potential for introducing contaminants into the tubes, necessitating post-forming cleaning operations. Furthermore, deformation of the tube walls raises the potential for excessive wall thinning, and therefore reduced strength. Finally, internal expansion methods are not well suited for use with heat exchangers formed with a serpentine tube.
From the above, it can be appreciated that it would be advantageous if an improved method were available for mechanically joining the tubes and fins of a heat exchanger. Such a method would preferably result in joint strength comparable to internal expansion methods, but rely entirely on an external expansion technique so as to avoid the disadvantages of internal expansion methods, including the potential for contamination and wall thinning. A preferred technique would also be well suited for use on heat exchanger designs incorporating a serpentine tube configuration.